Testing promise rejection with Mocha

promise-test.js

const { assert } = require('chai');

function isError(e) {
    if (typeof e === 'string') {
        return Promise.reject(new Error(e));
    }
    return Promise.resolve(e);
}


describe('Promise Testing', () => {
  // With done callback - 8 lines
  it('should test rejections using done', (done) => {
    aMethodThatRejects()
      .then(() => {
        done(new Error('Expected method to reject.'))
      })
      .catch((err) => {
        assert.isDefined(err);
        done();
      })
      .catch(done);
  });
  
  // With built in Promise support - 8 lines + helper
  it('should test rejections using catch', () => {
    return aMethodThatRejects()
      .then(() => {
        return Promise.reject('Expected method to reject.');
      })
      .catch(isError)
      .then((err) => {
        assert.isDefined(err);
      });
  });
});